1. Field of the Invention
The present disclosure relates to a power amplifier and relates to, for example, a high frequency power amplifier used for a wireless communication device or the like.
2. Description of the Related Art
In recent years, mobile communication device such as portable phones are used as information communication means not only for talking but also for transmitting and receiving various data such as mail and contents. Along with this situation, portable phones also have come to be equipped with a wireless LAN (Local Area Network) or the like.
As a modulation wave for high-speed data communication with a wireless LAN, the OFDM (Orthogonal Frequency Division Multiplexing) method is used. Because a system using the OFDM needs high modulation accuracy, a high frequency power amplifier is required to have high linearity. Therefore, the high frequency power amplifier is designed to be used in a linear amplification region in which output is sufficiently smaller than a maximum output.
However, if the number of transistors or a size of the transistor is increased in order to increase the maximum output of the high frequency power amplifier, thermal response of the amplifier becomes slow. For this reason, it takes some time since temperature of the amplifier starts to increase due to heat generation of the amplification transistor just after start-up of the amplifier until the temperature becomes stable. A gain and a phase of the amplifier change while the temperature is increasing, and these changes also can be a cause for fatal deterioration of an EVM (Error Vector Magnitude) in the OFDM.
In order to solve the above problem, a power amplifier as shown in FIG. 7 is proposed in International Publication WO2012/111274. The power amplifier shown in FIG. 7 includes capacitance element 501 whose first end is connected to control terminal 200 through which a control voltage is supplied, time constant control circuit 180 connected to a second end of capacitance element 501, discharge circuit 160 connected in parallel with capacitance element 501, and bias circuit 120 connected to time constant control circuit 180 and temperature compensation circuit 130.
When the control voltage rises, capacitance element 501 is charged, and a charge current caused by electric charge for the charging flows through resistor 192. This operation transiently increases a voltage output from temperature compensation circuit 130, and a bias for amplification transistor 104 can thus be transiently increased. Accordingly, a gain of the amplifier temporarily increases, and it is possible to shorten a time period until temperature fluctuation due to heat generated by amplification transistor 104 reaches an equilibrium state in the whole circuit. As a result, it is possible to reduce deterioration of the EVM caused by the temperature fluctuation due to the heat generated by amplification transistor 104 just after the start-up of the amplifier.